Nebulizers for analytical purposes are known in the art; see for example the papers by Denton, et al, Analytical Chemistry, Volume 44, February 1972, pp 241 ff; by Chester, Analytical Chemistry, Volume 52, September 1980, pp 638 ff and 1621 ff; Denton, U.S. Pat. No. 3,866,831; Olson, et al, U.S. Pat. No. 4,109,863; and Smith, Jr., et al, U.S. Pat. No. 4,361,401. Such nebulizers convert a liquid, for example, a liquid chromatography effluent, into an aerosol that is supplied to a gas-type detector, such as a flame photometric detector. One general prior art type of nebulizer is characterized as a pneumatic nebulizer wherein a liquid to be nebulized is shattered into droplets by supersonic gas flowing through an orifice. The orifice may be of the Venturi-type wherein liquid is sucked into a nubulizing region by a Bernoulli effect. Alternatively, the orifice can be of the Babbington-type wherein liquid is pumped across a nebulizing orifice. Another prior art nebulizer is an ultrasonic bath nebulizer wherein a volume of liquid is maintained in the nebulizer and nebulization occurs from the liquid surface. An additional prior art nebulizer is characterized as a flow nebulizer wherein a liquid flows onto a vibrating surface. A further type of prior art nebulizer includes a single droplet generator wherein the liquid to be nebulized is pumped through a vibrating capillary tube. The flow nebulizers are generally characterized by metal longitudinal or flexing oscillators or a glass surface covering a piezoelectric crystal, such as disclosed by Olson et al, or a glass or plastic diaphragm activated by coupling ultrasonic excitation through a liquid medium.
Pneumatic nebulizers, as contrasted to ultrasonic nebulizers, produce aerosols containing a wide range of droplet sizes; some of the droplets have a relatively large diameter. Pneumatic nebulizers work well with relatively high gas and liquid flows on the order of 10 liters per minute of gas and one cubic centimeter per minute of liquid. The typical gas analyzer is incapable of adequately handling an aerosol resulting from this nebulization. The pneumatic nebulizers exhibit severe problems in manufacture and alignment if attempts are made to design them to provide gas flow rates to an analyzer of less than one liter per minute. The efficiency of aerosol production by pneumatic nebulizers is relatively low and the resultant mist has a low droplet density.
Bath-type ultrasonic nebulizers have a relatively large volume of in-transit liquid, which results in unacceptable loss of resolution of the output of a liquid chromatography column or flow injection analyzer. The unacceptable loss of resolution or band broadening occurs for liquid flow rates of less than or equal to approximately 1 cubic centimeter per minute.
Ultrasonic nebulizers having flat crystals consume relatively large amounts of power, approximately 50 watts. This is undesirable because of the necessity to provide relatively expensive, complex and difficult to design amplifiers for radio frequency sources required to drive the piezoelectric crystal of such nebulizers. In the typical prior-art flat crystal ultrasonic nebulizer, the minimum achievable mixing volume is the volume of one liquid drop before nebulization, i.e., about 20 microliters. On the other hand, metallic resonators exhibit mechanical problems at frequencies above 100 KHz. Because drop size is related to oscillation frequency, with increasing frequency resulting in decreased drop size, such devices produce drops that are excessively large for many gas analyzers; the diameter of such drops is typically on the order of 50 microns.
Single droplet generators also produce aerosol drops having relatively large diameter and thus are incompatible with many gas analyzers. In addition, single droplet generators employ capillary tubes which are subject to plugging and therefore require extensive maintenance.
It is desirable in many analytical systems for a nebulizer to produce a dense aerosol of small, uniformly sized droplets from a liquid flowing at rates from 10 to 200 microliters per minute, as derived from a liquid chromatography column. The small, uniformly sized droplets must be capable of being mixed with a carrier gas having a flow rate as low as 30 cubic centimeters per minute. It is preferable for the nebulizer mixing volume to be on the order of 1 microliter so that the nebulizer can be used for supplying an aerosol from the effluent of a micro liquid chromatography column; such columns typically have an inner diameter of 1 millimeter. The low carrier gas flow, combined with a variable efficiency for the nebulizer, enables the aerosol to be supplied to a typical gas chromatography detector, such as a flame photometric detector, a flame ionization detector, or a mass spectrometer ion source. To minimize cost of the nebulizer, it is desirable for RF power supplied to a crystal to be minimized to a few watts. This enables amplifiers for supplying such power to be relatively low cost, and simple structures.
A further problem with prior nebulizers employing electric wave to pressure-wave transducers, e.g., piezoelectric crystals, is that such transducers have a tendency to change resonant frequency characteristics as a function of ambient conditions. In particular, as temperature changes, the resonant frequency of a piezoelectric crystal changes.